1. Field of the Invention
The present invention is useful in bonding low-energy plastic surfaces (substrates) to each other, for example, in construction, automotive applications, plastic fabricating, packaging, structural foam bonding, assembly of toys, etc. The novel adhesive composition described and claimed herein is effective in bonding low-energy plastic surfaces to each other which provides a tensile lap shear strength in excess of about 520 pounds per square inch gauge which comprises (1) an ethylene-vinyl ester copolymer having from about four to about 30 weight percent of a vinyl ester and a melt index of about 0.5 to about 75; (2) a tackifier selected from the group consisting of (a) a rosin, (b) a rosin ester, (c) a terpene resin, (d) a terpenephenolic resin and (e) a rosin-modified phenolic resin; and (3) a polyethylene wax having a molecular weight of about 750 to about 3000, a penetration of about 0.25 to about 1.5, a density of about 0.93 to about 0.97, a melting point of at least about 110.degree. C. and a linearity of at least about 70 percent, said novel adhesive composition having a Ring and Ball softening point of at least about 110.degree. C.
2. Description of the Prior Art
Ordinarily, adhesive bonding of low-energy plastic surfaces to each other requires some form of surface treatment prior to application of an adhesive to obtain high bond strengths between the surfaces so contacted. Low-energy plastic surfaces bonded to each other without prior surface treatment, such as by relatively weak, tacky adhesives, are inadequate for many purposes such as those requiring strengths in a range in excess of several hundreds or thousands of pounds per square inch gauge of bonded area.
For strong bonds involving low-energy plastic surfaces, some form of surface treatment or etching is required prior to application of an adhesive. Surface pretreatment of low-energy plastic surfaces, for example, involves either acid etching with sulfuric, nitric, chromic acids or mixtures thereof, or flame treatment, corona discharge treatment, treatment with plasma jets or low and high energy radiation.
Adhesive compositions which may be used for bonding low-energy plastic surfaces, but requiring surface treatment, include polysulfide-rubber modified epoxies, epoxy-polyamides, nitrile-phenolics and polyesters. These compositions, additionally, may require long hours of post-cure and/or clamping at temperatures substantially higher than room temperature. Often at least two of the components of these adhesive compositions must be premixed prior to application.
Consequently, a need exists for adhesive composition and much simpler process for bonding low-energy plastic surfaces to each other. In accordance with the present invention a novel hot melt adhesive composition is provided which requires minimal heating of the surfaces to be bonded together and maximum bond strength in a matter of minutes or a few hours without resort to elaborate postcuring procedures. Further, the adhesive composition of the present invention can be applied in its melt form to low-energy plastic surfaces in such a manner to provide rapid assembly of components and immediate handling of the assembled parts. Another advantage of the present invention is that the bonds can deliver strengths in excess of 520 pounds per square inch gauge under tension, preferably in excess of 550 pounds per square inch gauge.
Unlike the present invention, U.S. Pat. No. 3,560,420 to Tamura et al is directed to an adhesive containing ethylene-vinyl acetate, rosin, and polyethylene having a molecular weight in excess of about 5000 for bonding a resin film to wood.
U.S. Pat. No. 3,615,106 to Flanagan et al discloses an adhesive containing ethylene-vinyl acetate, a tackifier, polyethylene having a molecular weight in excess of 10,000 and a diluent for binding books.
U.S. Pat. No. 3,689,334 to Dermody discloses an adhesive mixture comprising an olefinic interpolymer, such as a copolymer of ethylene and vinyl acetate, a carboxylic functional resin, such as rosin, and a hydrocarbon wax. Although paraffin waxes are preferred, low molecular weight polyethylene can be used.
Japanese Patent Publication 51-47928 to Kobayashi et al, published Apr. 24, 1976, discloses a composition containing an ethylene-vinyl acetate copolymer, a terpene resin and a homopolymer or copolymer of olefins or diolefins having from two to four carbon atoms having an average molecular weight of 500 to 1000.